1. Field of the Invention
The present invention relates to an electrooptic device which is applicable to various optical information processing devices such as an optical reading/writing device etc.
Also, more particularly, the present invention relates to an electrooptic lens which is used in an optical device such as an optical head of an optical information reading/writing apparatus.
2. Description of the Related Art
An electrooptic crystal is one of the important materials used in an optical device. An example of the usage of the electrooptic crystal is disclosed in the electromagnetic field theory research report of the Electric Society, 1985 (EMT-85, No. 16-20, P25-33). The report discloses a theoretical analysis of an electrooptic crystal arrangement wherein a minute distributed index type micro lens plate less than 250 .mu.m is formed on a crystal plate of LiNbO.sub.3 by a proton exchange method and an electrode is formed thereon wherein a voltage is applied to the electrode so as to change the focal length of the lens by changing the refractive index thereof due to the electrooptic effect.
Also, a document "Total internal reflection lens" of "APPLIED OPTICS/Vol. 24, No. 9/1, May 1985" discloses a total reflection lens using an electrooptic crystal of LiTaO.sub.3. The document discloses an arrangement wherein a minute electrode array is formed on an X crystal face or Z face of an electrooptic crystal of LiTaO.sub.3 and wherein a beam waist position of a laser beam is changed in such a way that a convergent incident laser beam is irradiated on the X or Z face of the crystal at an incident angle of 1.4.degree. with a beam diameter of about 20 .mu.m and that a voltage is applied to the electrode array so as to change the refractive index of the crystal lens due to the electrooptic effect whereby the position of the beam waist of the incident laser beam is shifted.
However, with respect to the former arrangement wherein a minute distributed index micro lens plate is formed on the electrooptic crystal of LiNbO.sub.3 by the proton exchanging method, there are some disadvantages as follows.
First, it is not easy to manufacture the lens in accordance with such an arrangement.
Second, the incident beam diameter has to be less than 250 .mu.m which limits the applicable optical devices.
Third, theoretically, the focal position is shiftable only within a range of 200 .mu.m.
Fourth, the convergent characteristic of the lens arrangement is unclear in practical use thereof since the arrangement is analyzed only by the theoretical calculation as yet.
Fifth, the arrangement does not have a deflection function.
Also, the latter arrangement of the total reflection lens using the electrooptic crystal of LiTaO.sub.3 has the following disadvantages.
First, the incident laser beam has to be arranged substantially in parallel (1.4.degree.) with the crystal face (reflection surface).
Second, the incident beam has to be converged so that the diameter thereof on the reflection surface becomes less than a predetermined value, for example 20 .mu.m. Therefore, it becomes necessary to prepare a special condenser lens system for condensing the beam, which bulks and complicates the optical device structure.
Third, the arrangement does not have a deflection function.
In order to cope with some of the above-mentioned disadvantages, an electrooptic lens is proposed which is arranged in such a way that a voltage is applied to a pair of electrodes having an electrooptic material such as PLZT disposed therebetween so that a refractive index distribution is generated in the material due to the electrooptic effect so that the electrooptic material functions as a lens. Such an electrooptic lens has a relatively simple structure and can be manufactured easily. Also, the incident beam condition required for the above-mentioned electrooptic lens is not so strict. However, the electrooptic lens does not have a function of deflection.
In other words, there has not been an electrooptic device having a deflection function as well as a lens function.
Of course, it is possible to combine an optical deflector with a variable-focal-length lens using the electrooptic crystal as disclosed in the above-mentioned documents. However, in accordance with such a combination arrangement, it is necessary to prepare an individual lens and an individual deflector. Thus, it has not been realized an optical device having a lens function and a deflection function which device is formed as one unit formed from one body.
Also, the electrooptic lens mentioned above is applicable only to a beam having a special sectional shape, which impairs the matching characteristic thereof with the optical deflector to be combined with.
The convergent characteristic of the electrooptic lens mentioned above is also insufficient so that aberration is generated in the image through the lens.